No abstract

A ouabain sensitive inward current occurs in Xenopus oocytes in Na + and K + -free solutions. Several laboratories have investigated the properties of this current and suggested that acidic extracellular pH (pH o ) produces a conducting pathway through the Na + /K + pump that is permeable to H + and blocked by [Na + ] o . An alternative suggestion is that the current is mediated by an electrogenic H + -ATPase. Here we investigate the effect of pH o and [Na + ] o on both transient and steady-state ouabainsensitive current. At alkaline or neutral pH o the relaxation rate of pre-steady-state current is an exponential function of voltage. Its U-shaped voltage dependence becomes apparent at acidic pH o , as predicted by a model in which protonation of the Na + /K + pump reduces the energy barrier between the internal solution and the Na + occluded state. The model also predicts that acidic pH o increases steady-state current leak through the pump. The apparent pK of the titratable group(s) is 6, suggesting that histidine is involved in induction of the conductance pathway. 22 Na efflux experiments in squid giant axon and current measurements in oocytes at acidic pH o suggest that both Na + and H + are permeant. The acid-induced inward current is reduced by high [Na + ] o , consistent with block by Na + . A least squares analysis predicts that H + is four orders of magnitude more permeant than Na + , and that block occurs when 3 Na + ions occupy a low affinity binding site (K 0.5 = 130 ± 30 mM) with a dielectric coeffcient of 0.23 ± 0.03. These data support the conclusion that the ouabain-sensitive conducting pathway is a result of passive leak of both Na + and H + through the Na + /K + pump.

No abstract

No abstract